1. Field of the Invention
The invention relates to catalyst support structures having a passivated or hydrophobic layer, and particularly cordierite or aluminum titanate catalyst support structures/diesel filters having a hydrophobic external coating.
2. Discussion of the Related Art
Recently much interest has been directed towards the diesel engine due to its efficiency, durability and economical aspects. However, diesel emissions have come under attack both in the United States and Europe, for their harmful effects on the environment and on humans. As such, stricter environmental regulations will require diesel engines to be held to the same standards as gasoline engines. Therefore, diesel engine manufacturers and emission-control companies are working to achieve a diesel engine which is faster, cleaner and meets the most stringent of requirements under all operating conditions with minimal cost to the consumer.
Similarly to conventional engines, the exhaust gas discharged from diesel engines needs to be purified of nitrogen oxides (NOx). However, unlike conventional engines which simply employ three-way catalysts, the diesel engine, which is a partial lean burn gasoline engine producing exhaust gas with an excess amount of oxygen, cannot employ only three-way catalysts because in order for these types of catalysts to function properly they require conditions where the air-fuel ratio is substantially stoichiometric.
NOx traps appear to be a leading candidate for exhaust purification in diesel engines. NOx traps are similar to three-way catalysts, in that they are made of a catalyst support and a catalyst coating the support, with the difference existing in that NOx traps include an additional component in the catalyst coating which stores the trapped NOx. As the NOx-adsorbing component used in the NOx-adsorbing catalyst, there are known alkali metals such as K, Na, Li, Cs and the like; alkaline earth metals such as Ba, Ca and the like; and rare earth elements such as La, Y and the like.
NOx-adsorbing catalysts are generally formed by loading a catalyst layer containing the above-mentioned NOx-adsorbing component, on a monolithic catalyst carrier (monolith) composed of an oxide type ceramic (e.g. cordierite) or a metallic material (e.g. Fe—Cr—Al alloy). In the industry cordierite (2MgO-2Al2O3-5SiO2) has been the cost-effective ceramic material of choice for NOx-carrier/supports for heavy duty vehicles due to its combination of excellent thermal shock resistance, filtration efficiency, and durability under most operating conditions. Recently, aluminum titanate-based ceramic materials have been gaining acceptance as another suitable NOx-carrier/support material.
Another significant challenge in lowering diesel emissions is controlling the levels of diesel particulate material present in the diesel exhaust stream. In 1998 diesel particulates were declared a toxic air contaminant by the California Air Resources Board. Legislation has been passed that regulates the concentration and particle size of diesel particulate pollution originating from both mobile and stationary sources.
Diesel particulate material is mainly carbon soot and one way of removing the carbon soot from the diesel exhaust is through diesel traps. The most widely used diesel trap is the cordierite ceramic diesel particulate filter (DPF) which filters the diesel exhaust by capturing the soot on the porous walls of the filter structure. The DPF is designed to provide for nearly complete filtration of soot without significantly hindering the exhaust flow and also, in some instances, like the NOx cordierite catalyst supports, includes a catalyst material; e.g., an oxidation catalyst. Again, aluminum titanate-based ceramic materials (AT) are finding applications as the DPF material.
Regardless of whether the cordierite or AT monolith is in the form of a catalyst carrier substrate or as a DPF, coldset ceramic cement (preferably cordierite) has long been used to form the exterior skin of the cordierite monolith. The coldset ceramic cement is mixed and applied to a fired, contoured substrate and the wet skin is afterwards allowed to dry either under ambient conditions or by convective or microwave drying at elevated temperatures. The substrate with exterior skin layer is then ready to receive a catalyst coating and any further downstream processing required.
As diesel filtration and catalyst products evolve and product performance demands change the catalyst coating process and chemistry, these changes are likely to result in incompatibilities between the catalyst coating process and the ceramic monoliths with exterior skin layers; whether in the form of NOx support substrates or in the form of DPFs. One example of an incompatibility between monolith with applied exterior skin layers and catalyst coating processes is that the monolith and/or skin can exhibit excessive skin or coating cracking or spalling during or following the subsequent catalyst coating steps, due to absorption of coating liquid by the skin. Other problems relating to monolith performance that have been observed following the application of catalyst coating media to ceramic monoliths include a substantial increase of the CTE (coefficient of thermal expansion) and/or increase of the elastic modulus of the monolith, both of these leading to decreased thermal shock performance (large axial and radial cracking) in the end use application. In addition, the catalyst coating can reduce the strength of the monolith. Catalyst coatings are well known in the art and can include compounds comprising cerium, platinum, rhodium, palladium, colloidal alumina, etc.
It would be considered an advance in the art to provide a ceramic monolith incorporating a coldset skinning cement that is impervious to a wide range of catalyst coating chemistries and associated processes, thus resulting in a reduction in the number of ceramic monolith product failures that occur as a result of incompatibility between substrate skin materials and the catalyst coating process. Another advance would be to provide a ceramic monolith wherein the catalyst coating does not substantially change the CTE, modulus, strength or thermal shock performance of the monolith.